It would appear that the Slovak-Hungarian "Blood Countess" Elizabeth Báthory may have been on to something: Researchers have shown that it is possible to reverse cognitive decline in old mice by injecting them with blood from the young. Elderly mice who were given transfusions of young blood were shown to exhibit improved learning skills and memory — and at a level comparable to much younger mice. Should the same effect apply to humans, it could represent a novel way to treat neurodegenerative disorders like Alzheimer's and Parkinson's.

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A primary reason why we experience cognitive decline as we get older is due to our decreased production of neural stem cells — what ends up causing fewer connections between brain cells. Exercise has been known to help, but its effects are limited.

But Saul Villeda of Stanford University has shown that the production of these stem cells may have something to do with the quality of our blood.

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He first made this discovery two years ago when he injected the blood of an older mouse into a younger one and vice versa. Villeda did this by connecting the circulatory systems of two mice so that their blood could mix (what's called heterochronic parabiosis). Soon after the transfusion he noticed that the younger mouse's brain started to age much more rapidly. And when he analyzed the older mouse, he observed that the number of stem cells had increased. His study led to a research paper that was later published in Nature.

More recently, Villeda's experiments have sought to determine whether or not this effect translates to behavioral changes as well. In the new study, he performed a similar transfusion and put the mice into a water maze where they were required to perform memory tasks. He discovered that the older mice did almost as well as mice who were four to six months old. As for the older, untreated mice, they made lots of mistakes and continually swam down blind alleys.

In terms of what's happening, the researchers say the young blood is likely reversing the aging process by topping up levels of key chemical factors that normally decline in the blood as we age. Speaking to the Guardian, Villeda noted that, after the transfusion, "all of a sudden you have all of these plasticity and learning and memory-related genes that are coming back." But as to which exact factors are causing the effect, the researchers are not sure — there are hundreds of thousands of candidate factors.

Looking ahead, Villeda says his team's insights could result in genuine rejuvenation therapies for people dealing with cognitive decline. The first step, however, is to determine whether or not this effect translates to humans.

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Villeda presented these results at the annual meeting of the Society for Neuroscience in New Orleans on October 17, 2012. An earlier version of his research appeared in Nature last year.